Multilayer coating film formation process

ABSTRACT

The present invention provides a process of a forming multilayer coating film characterized by that in a process of forming a multilayer coating film by successively coating with an undercoat paint (A), an intermediate paint (B) and a topcoat paint (C) on a substrate, 
     (1) to use a liquid thermocurable paint containing 0.1-30 parts by weight of aluminium powder and 1-200 parts by weight of titanium oxide pigment per 100 parts by weight of a thermosetting resin composition and the hiding film thickness of its coating film being less than 25 μm as said intermediate paint (B), 
     (2) to use a solid color paint, a metallic paint or an interference pattern paint as said topcoat paint (C), and 
     (3) to coat said topcoat paint (C) after curing by heating of the coating film of said intermediate paint (B). By this process it is possible to make an intermediate coating film thinner and to form a multilayer coating film with excellent smoothness.

This application is a 371 application of PCT/JP96/03278 filed Nov. 8,1996.

TECHNICAL FIELD

The present invention relates to a process of forming a multilayercoating film with excellent smoothness and with thinner intermediatecoating film without deteriorating the coating film properties by usingan intermediate paint with a specific pigment composition and by coatinga topcoat paint after curing of the coating film of said intermediatepaint, when a multilayer coating film is formed by successive coatingsby an undercoat paint, an intermediate paint and a topcoat paint.

BACKGROUND TECHNOLOGY

A process of forming a multilayer coating film by successively coatingwith an undercoat paint (such as an electrodeposition paint), anintermediate paint and a topcoat paint is known. Concerning theintermediate paint, however, it is necessary to coat to a thick coatingfilm of usually more than 30 μm (as a cured coating film) in order tohide the undercoat layer and to maintain the coating film properties.Therefore, it is desired to lower the coating cost of the wholemultilayer coating film by making the intermediate coating film thinnerwithout deteriorating the hiding properties and the coating filmproperties.

The present inventors had been conducting an intensive research to solvethese problems and as a result they found that in a process ofsuccessive coatings by an undercoat paint, an intermediate paint and atopcoat paint it is possible to make the intermediate coating filmthinner with improved undercoat hiding properties and chippingresistance of the intermediate coating film and, in addition, withimproved smoothness of the topcoat surface by using a thermocurablepaint containing both components of aluminium powder and titanium oxidepigment as the intermediate paint and by coating a topcoat paint aftercuring of the intermediate coating film, and completed the presentinvention.

DISCLOSURE OF THE INVENTION

Thus the present invention provides a process of forming a multilayercoating film characterized by that in a process of forming a multilayercoating film by successively coating a substrate with an undercoat paint(A), an intermediate paint (B) and a topcoat paint

(1) to use a liquid thermocurable paint containing (C), 0.1-30 parts byweight of aluminium powder and 1-200 parts by weight of titanium oxidepigment per 100 parts by weight of a thermosetting resin composition andthe hiding film thickness of its coating film being less than 25 μm assaid intermediate paint (B),

(2) to use a solid color paint, a metallic paint or an interferencepattern paint as said topcoat paint (C), and

(3) to coat said topcoat paint (C) after curing by heating of thecoating film of said intermediate paint (B).

The process of forming a multilayer coating film of the presentinvention (hereinafter referred to as “the process”) is described morespecifically hereinbelow.

Undercoat paint (A):

Undercoat paint (A) is used to give anticorrosivity, adhesivity etc. bydirectly coating a substrate made of metal or plastics. In the processany usual undercoat paint can be used without special restriction, onlyif it suits to these purposes. As a substrate applicable to saidundercoat paint outer panels of automobile are particularly preferable.It is usually desirable to suitably conduct rust removal, washing andchemical treatments of the substrate previously.

If a substrate is metalbacked or has an electroconductive surface, acationic electrodeposition paint is preferable as an undercoat paint. Asa cationic electrodeposition paint there can be used a per se known onecomprising an aqueous solution or aqueous dispersion of a salt ofcationic high polymer compound, compounded, as necessary, withcrossliking agent, pigment and various additives and its sort is notspecially restricted. As a cationic high polymer compound there can bementioned, for example, acrylic resin or epoxy resin having crosslinkingfunctional groups in which cationic groups such as amino groups areintroduced. They can be made water-soluble or water-dispersible throughneutralization with an organic acid or an inorganic acid. As acrosslinking agent to cure these high polymer compounds blockpolyisocyanate compounds, alicyclic epoxy resins etc. can be preferablyused.

Electrodeposition coating can be conducted by dipping metallicsubstrates such as outer panels of automobile or bumpers as a negativeelectrode in a bath of said cationic electrodeposition paint and bydepositing the paint on said substrate by sending an electric currentbetween it and a positive electrode under the usual conditions. Thethickness of the formed electrodeposition coating film is preferableusually in a range of 10-40 μm based upon a cured coating film, and acoating film can be cured by crosslinking through heating at about 140to about 220° C. for about 10 to about 40 minutes. In the process it ispreferable to coat an intermediate paint after curing saidelectrodeposition coating film. Optionally, however, an intermediatepaint can be coated during the latter is in the uncured state.

Intermediate Paint (B):

In the process a liquid thermocurable paint containing 0.1-30 parts byweight of aluminium powder and 1-200 parts by weight of titanium oxidepigment per 100 parts by weight of a thermosetting resin composition andthe hiding film thickness of its coating film being less than 25 μm asthe intermediate paint (B),

Using both aluminium powder and titanium oxide pigment in theintermediate paint (B) increases the hiding power of the coating filmand make it possible to sufficiently hide the undercoat surface with acured coating film of less than 25 μm, especially with a thin film of10-25 μm and thus can achieve making the intermediate coating filmthinner.

Thermosetting resin composition which is used as a vehicle component insuch an intermediate paint (B) consists fundamentally of a base resinand a cross-linking agent or a self-crosslinking type resin. As a baseresin there can be mentioned, for example, acrylic resin, polyesterresin, alkyl resin etc. having more than 2 crosslinking functionalgroups such as hydroxyl group, epoxy group, isocyanate group, carboxylgroup etc. in the molecule. As a crosslinking agent there can be used,for example, amino resins such as melamine resin, urea resin etc.,polyisocyanate compounds which may be blocked, compounds containingcarboxyl groups etc. As the above-mentioned self-crosslinking type resinthere can be mentioned, for example, resins containing more than 2alkoxysilane groups in the molecule, resins containing a carboxylgroup(s) and a hydroxyl group (s) in the molecule, resins containing ahydroxyl group (s) and an isocyanate group(s) which may be blocked etc.These resins are based upon, for example, vinyl resin, acrylic resin,polyester resin, urethane resin etc.

As aluminium powder to be compounded in the intermediate paint (B)aluminium powder whose average particle diameter is generally less than40 μm, preferably less than 10 μm, and more preferably in a range of 3-7μm is suitable. Particularly, if fine powder of an average particlediameter of less than 10 μm is used, the formed intermediate coatingfilm itself has no brilliance. “Average diameter” here means a mediandiameter measured by laser diffraction scattering (LA-500). The maincomponent of this aluminium powder is metallic aluminium, whose surfacemay be treated with a silane coupling agent or the like.

As titanium oxide pigment to be compounded in the intermediate paint (B)according to the process those which are per se known as pigments forpaint can be used. Their average particle diameter is preferable to begenerally 5 μm. Moreover, the surface of said titanium oxide pigment maybe treated with alumina, silica etc.

Concerning compounding amount of aluminium powder and titanium oxidepigment per 100 parts by weight of a thermosetting resin composition (assolid content) aluminium powder can be in a range of 0.1-30 parts byweight, preferably 0.5-20 parts by weight and more preferably 1-7 partsby weight and titanium oxide pigment can be in a range of 1-200 parts byweight, preferably 40-160 parts by weight and more preferably 80-120parts by weight. Furthermore, aluminium powder is preferably used in arange of 1-15 parts by weight, preferably 1.5-10 parts by weight andmore preferably 2-7 parts by weight per 100 parts by weight of titaniumoxide pigment.

It is indispensable that the intermediate paint (B) which is used in theprocess contains both aluminium powder and titanium oxide pigment. Totalcompounding amount of both pigments can be selected to be an amountwhich enables to make the hiding film thickness of the coating film tobe formed by using said paint (B) less than 25 μm, particularly lessthan 10-25 μm (as cured coating film). “Hiding film thickness” heremeans the minimum film thickness of a coating film through which thecolor of the surface to be coated cannot be recognized and specificallymeans the minimum film thickness of a coating film coated on a platewith black and white checkered pattern through which black and whitecannot be discriminated by the naked eye. In the process compoundingboth aluminium powder and titanium oxide pigment in combination atspecified amounts enables to make the hiding film thickness of a coatingfilm a thin film of less than 25 μm. In other words, a thin film of evenless than 25 μm can sufficiently hide the color of the ground. Withoutany of these both components it is difficult to hide with such a thinfilm.

Intermediate paint (B) can be prepared by mixing and dispersing theabove-mentioned thermosetting resin composition, aluminium powder andtitanium oxide pigment in a solvent such as an organic solvent and/orwater. Furthermore other color pigments than the above-mentionedaluminium powder and titanium oxide pigment, extender pigment,antisettle agent etc. can be suitably compounded, as necessary.

Said intermediate paint (B) is preferably coated in a film thickness ofless than 25 μm, particularly in a range of 10-25 μm based upon a curedcoating film on the cured or uncured undercoat surface by means ofelectrostatic coating, air spray, airless spray etc.

In the process a topcoat paint (C) mentioned below is coated after thecoating film of the intermediate paint (B) has been cured by heating.Curing by heating of the coating film of the intermediate paint (B) canbe conducted, for example, by heating said coating film at temperaturesof about 140 to about 200° C. for about 10 to about 40 minutes.

Topcoat paint (C):

According to the present invention solid color paint (C-1), metallicpaint (C-2) or interference pattern paint (C-3) is coated as a topcoatpaint on the cured coating surface of the intermediate paint(B). All ofthese topcoat paints are desirably of thermocurable type.

First of all, as a solid color paint (C-1) there is used preferably aliquid thermocurable paint containing a thermosetting resin compositionand a color pigment as main components and substantially not containingmetallic pigment or interference color pigment.

Thermosetting resin composition which is used in a color paint (C-1)consists fundamentally of a base resin and a crosslinking agent or aself-crosslinking type resin. As a base resin there can be mentioned,for example, acrylic resin, polyester resin, alkyl resin etc. havingmore than 2 crosslinking functional groups such as hydroxyl group, epoxygroup, isocyanate group, carboxyl group etc. in the molecule. As acrosslinking agent there can be mentioned, for example, amino resinssuch as melamine resin, urea resin etc., polyisocyanate compounds whichmay be blocked, compounds containing carboxyl groups etc. Further, asthe above-mentioned self-crosslinking type resin there can be mentioned,for example, resins containing more than 2 alkoxysilane groups in themolecule, resins containing a carboxyl group(s) and a hydroxyl group (s)in the molecule, resins containing a hydroxyl group (s) and anisocyanate group(s) which may be blocked etc. These resins are basedupon, for example, vinyl resin, acrylic resin, polyester resin, urethaneresin etc.

Color pigment which can be compounded in a solid color (C-1) does notsubstantially contain metallic pigment or interference pigment but is acomponent to give solid color to the multi layer coating film which isformed according to the process of the present invention and usualorganic or inorganic color pigments for paint can be used. Specificallythere can be mentioned, for example, inorganic pigments such as titaniumoxide, zinc oxide, carbon black, cadmium red, molybdenum red, chromeyellow, chrome oxide, Prussian Blue, Cobalt Blue; organic pigments suchas azo pigment, phthalocyanine pigment, quinacridone pigment,isoindoline pigment, threne type pigment, perylene pigment etc. Thesepigments desirably have an average particle diameter of generally lessthan 5 μm.

Compounding amount of these color pigments can be freely selectedaccording to the coloring power of the pigment itself and the purpose.It can be in a range of generally 0.5-200 parts by weight, preferably1-150 parts by weight per 100 parts by weight of the thermosetting resincomposition and an amount which allows the hiding film thickness of thecoating film to be formed to be less than 50 μm, particularly less than40 μm as a cured coating film.

Solid color paint (C-1) can be prepared by mixing and dispersing theabove-mentioned components in a solvent such as an organic solventand/or water. In said paint, as necessary, extender pigment, antisettleagent etc. can be further compounded suitably.

As a metallic paint (C-2) there can be used preferably a liquidthermocurable paint containing a thermosetting resin composition, aboutwhich is mentioned above in the item of a solid color paint (C-1), and ametallic pigment as main components. Metallic pigment which iscompounded in this metallic paint is a scale-like particle pigment ofmetal or metal oxide having a glittering brilliancy and specificallythere can be mentioned, for example, aluminium flake, mica-like ironoxide etc. These scale-like pigment particles can have an averageparticle diameter of generally more than 10 μm, preferably in a range of10-50 μm and more preferably in a range of 15-40 μm. Compounding amountof these metallic pigments can be in a range of generally 0.1-20 partsby weight, preferably 3-10 parts by weight per 100 parts by weight ofthe thermosetting resin composition and an amount which allows thehiding film thickness of the coating film to be formed to be less than50 μm, particularly less than 30 μm as a cured coating film.

Metallic paint (0-2) can be prepared by mixing and dispersing theabove-mentioned components in a solvent such as an organic solventand/or water. In said paint, as necessary, extender pigment, colorpigment, antisettle agent etc. can be compounded suitably. As a metallicpaint (C-2) contains metallic pigment with relatively large particlediameter, the coating film of said metallic paint itself shows aglittering brilliancy.

As an interference pattern paint (C-3) there is used preferably a liquidthermocurable paint containing a thermosetting resin composition, aboutwhich is mentioned above in the item of a solid color paint (C-1), andan interference pigment as main components. As an interference pigmentwhich is compounded in this interference pattern paint, scale-like mica,whose surface is covered with metal oxide such as titanium oxide, ironoxide etc., so-called interference mica, is particularly preferable.Covering thickness of metal oxide on this interference mica is desirablymore than 200 nm based upon an optical thickness and more than 80 nmbased upon a geometrical thickness. If said covering thickness is lessthan mentioned above, it is not preferable because the interferenceaction by light generally lowers. Said interference pigment can have anaverage particle diameter of generally more than 10 μm, preferably in arange of 10-50 μm and more preferably in a range of 15-40 μm.

Compounding amount of said interference pigment can be in a range ofgenerally 1-100 parts by weight, preferably 5-50 parts by weight per 100parts by weight of the thermosetting resin composition.

Interference pattern paint (C-3) can be prepared by mixing anddispersing the above-mentioned components in a solvent such as anorganic solvent and/or water. In said paint, as necessary, colorpigment, metallic pigment, extender pigment, antisettle agent etc. canbe compounded suitably.

The above-mentioned topcoat paints (C) are preferably coated in a filmthickness in a range of 10-60 μm, particularly in a range of 20-35 μmbased upon a cured coating film on the cured intermediate coatingsurface by means of electrostatic coating, air spray, airless spray etc.

The coating film of the above-mentioned top-coat paints (C) can becured, for example, by heating at temperatures of about 120 to about180° C. for 10-40 minutes.

Clear paint (D):

In the process, A clear paint (D) may be coated, as necessary, on thecoating surface of the topcoat paint (C) of the multilayer coating filmformed as mentioned above. The clear paint (D) can be coated on thecoating surface of the topcoat paint (C) formed as mentioned above inthe cured or uncured state.

As a clear paint (D), there can be preferably used a liquid paintcomprising a thermosetting resin composition and a solvent as maincomponents, and, as necessary, color pigment, metallic pigment,interference pigment, ultraviolet absorber and other additives for paintto such an extent as not to deteriorate the transparent feeling of thecoating film.

The above-mentioned thermosetting resin composition consistsfundamentally of a base resin and a crosslinking agent, or aself-crosslinking type resin. As a base resin there can be mentioned,for example, acrylic resin, polyester resin, alkyl resin, urethane resinetc. having more than 2 crosslinking functional groups such as hydroxylgroup, epoxy group, isocyanate group, carboxyl group etc. in themolecule. As a cross-linking agent there can be mentioned, for example,melamine resin, urea resin, polyisocyanate compounds which may beblocked, compounds containing carboxyl groups etc. Further, as aself-crosslinking type resin there can be mentioned, for example, resinscontaining more than 2 alkoxysilane groups in the molecule, resinscontaining a carboxyl group(s) and a hydroxyl group (s) in the molecule,resins containing a hydroxyl group (s) and an isocyanate group(s) whichmay be blocked, etc. These resins are based upon, for example, vinylresin, acrylic resin, polyester resin, urethane resin etc.

As a solvent, an organic solvent and/or water can be used. By dissolvingor dispersing the above-mentioned thermosetting resin composition andother components in such a solvent, a clear paint (D) can be prepared.

The clear paint (D) can be coated on the uncured or cured coatingsurface of the topcoat paint (C) formed as mentioned above by means ofelectrostatic coating, air spray, airless spray etc. Its film thicknessis preferably in a range of 10-60 μm and particularly 20-50 μm basedupon a cured coating film. The coating film itself of said clear paint(D) can be cured by crosslinking at temperatures of about 120 to about180° C. for 10-40 minutes.

According to the above-mentioned process of forming a multilayer coatingfilm of the present invention, for example, there are obtained theeffects mentioned below:

(1) The cost of the whole multilayer coating film can be lowereed,because the film thickness of the intermediate coating film can be madethinner (less than 25 μm, preferably 10-20 μm) than before (usually morethan 30 μm).

(2) The smoothness of the topcoat coating film is excellent, because thetopcoat paint is coated after the intermediate coating film has beencured by heating.

(3) As the intermediate paint has an excellent hiding properties of theground, the color stability of the topcoat coating film is good evenwhen coated with a thin coating film and the color design of the topcoatcoating film can be freely changed according to the purposes.

(4) Formed multilayer coating film has an excellent chipping resistance.

EXAMPLE

The process of the present invention is described more specifically bymeans of examples and comparative examples as follows:

I. Sample

(1) Cationic electrodeposition paint (A)

“ELECTRON9400HB” (made by Kansai Paint; trade name; epoxy resinpolyamine block isocyanate compound type)

(2) Intermediate paint (B)

Organic solvent type paints comprising polyester resin, melamine resin,fine aluminium powder and titanium oxide pigment in the ratios shown inthe following Table 1. Compounding amount of each component in Table 1is the solid content ratio by weight.

TABLE 1 intermediate paint (B) B-1 B-2 B-3 B-4 B-5 Polyester resin (*1) 65  70  75  70  70 Melamine resin (*2)  35  30  25  30  30 Finealuminum powder (*3)  5  2  2 —  2 Titanium oxide pigment (*4) 120 100 80  80 — Iron oxide pigment (red) (*5)  2  2  2  2  2 Hiding filmthickness (μm) (*6)  11  13  15 100 150 (*1): Polyester resin ofphthalic anhydride-hexahydro-phthalic anhydride type (number averagemolecular weight: about 4000, hydroxyl group value: 82, acid value: 7)(*2): U-Van28-60 (made by Mitsui-Toatsu Chemicals; trade name) (*3):K-9800 (made by Asahi Chemical; trade name), average paraticle diameter:5-6 μm (*4): TITANJR701 (made by Teikoku Kako; trade name), averageparaticle diameter: 0.3-0.6 μm (*5): KNO-W iron oxide (made by TodaKogyo; trade name), average paraticle diameter: 0.2-0.5 μm (red solidcolor pigment) (*6): The minimum film thickness (μm) of a coating filmcoated on a black and white plate with checkered pattern through whichblack and white cannot be discriminated by the naked eye was measured.

(3) Topcoat paint (C)

Organic solvent type paints comprising acrylic resin, melamine resin,solid color pigment or metallic pigment in the ratios shown in thefollowing Table 2.

Compounding amount of each component in Table 2 is the solid contentratio by weight.

TABLE 2 Topcoat paint (C) C-1 C-2 C-3 Acrylic resin (*7)  65  70  75Melamine resin (*8)  35  30  25 Titanium white pigment (*9)  80 — —Carbon black (*10)  0.2 — — Interference pigment (*11) —  9  9 Hidingfilm thickness (μm) (*6) 100< 100< 100< (*7): Acrylic resin of methylmethacrylate type with number average molecular weight of about 2000,hydroxyl group value of 70 and acid value of 8 (*8): U-Van28-60 (made byMitsui-Toatsu Chemicals; trade name) (*9): TITANCR93 (made by IshiharaSangyo; trade name) (*10): CarbonFW200 (made by DEGUSSA; trade name)(*11): Exterior Highlight Blue (made by Mahl; trade name; averageparaticle diameter: 14-18 μm)

(5) Clear Paint (D)

“Magicron Clear” (made by Kansai Paint; trade name; acrylicresin-melamine resin type; organic solvent type)

II. Examples and comparative examples

Multilayer coating films were formed by coating, using theabove-mentioned samples and according to the coating procedures shown inTable 3, followed by curing by heating. In Table 3 the results of theperformance tests of the multilayer coating films are mentioned, too.

TABLE 3 Examples Comparative examples 1 2 3 1 2 3 Electrodepositionpaint (A) Heating condition 170° C., 30 min. Intermediate paint B-1 B-2B-3 B-4 B-5 B-1 Drying condition 140° C., 30 min. room temp., 5 min.Topcoat paint C-1 C-2 C-3 C-1 C-2 C-1 Drying condition 140° C., roomtemp., 140° C., room temp., 140° C., 30 min. 5 min. 30 min. 5 min. 30min. Clear paint — D — D — Heating condition — 140° C., 30 min. — 140°C., — 30 min. Results of performance test Smoothness ◯ ◯ ◯ ◯ ◯ XFinishing appearance ◯ ◯ ◯ X X Δ Metallic feeling — ◯ ◯ — Δ — Chippingresistance ◯ ◯ ◯ Δ Δ ◯

The cationic electrodeposition paint (A) was painted byelectrodeposition on a steel plate, which had been degreased and treatedwith zinc phosphate, to the film thickness of 20 μm according to theusual method and the coating film was cured by heating at 170° C. for 30minutes. On said electrodeposition coating surface an intermediate paint(B-1)-(B-5) was coated so that the film thickness would be 25 μm and theintermediate coating film was cured by heating at 140° C. for 30 minutesin Examples 1-3 and in Comparative example 1 and 2, while it was keptstanding at room temperature for 5 minutes in Comparative example 3.Then on the intermediate coating surface a topcoat paint (C-1)-(C-3) wascoated using a minibell type rotary electrostatic coater under theconditions of output 150 cc, rotation number 50000 rpm, shaping pressure1 kg/cm², gun distance 30 cm, booth temperature 20° C., booth humidity75%. Coating film thickness was 15-25 μm. After said topcoat paint hadbeen kept standing in the booth for 5 minutes, the coating film of thetopcoat paint (C) was cured by heating at 140° C. for 30 minutes inExample 1 and in Comparative example 1 and 3. On the other hand, inExamples 2 and 3 and Comparative example 2, a clear paint (D) was coatedon the uncured coating surface of the topcoat paint (C) using a minibelltype rotary electrostatic coater under the conditions of output 300 cc,rotation number 40000 rpm, shaping pressure 5 kg/cm², gun distance 30cm, booth temperature 20° C., booth humidity 75%. Coating film thicknesswas 45-50 μm. After being kept standing at room temperature for 3minutes after coating, the double layer coating film consisting of theabove-mentioned topcoat paint (C) and clear paint (D) was simultaneouslycured by heating at 140° C. for 30 minutes using a hot air circulationtype drying furnace.

Coating film performance test methods and evaluation standards are asfollows:

Smoothness: Visual evaluation.

◯: good, Δ: a little face roughening,

X: remarkable face roughening.

Finishing appearance: Color floating and hiding properties are visuallyevaluated.

◯: good, Δ: fairly good, X: no good.

Metallic feeling: Visual evaluation about metallic mottling etc.

◯: good, Δ: fairly good, X: no good.

Chipping resistance: Using Gravelometer (Made by Q Panel) as a testingmachine, a shock is given to a coating film by blowing 500 g of No.7crushed stones by an air pressure of 3 kg/cm² at 20° C. onto the coatingsurface at an angle of 45°. Then an adhesive tape is stuck on saidcoating surface, and the state of peeling-off of the coating film aroundthe crack caused by the shock is examined, after rapidly peeling-off theadhesive tape.

◯: No or little peeling-off of the coating film around the crack isobserved.

Δ: Peeling-off of the coating film around the crack is clearly observed.

X: Peeling-off of the coating film around the crack is remarkablyobserved.

What is claimed is:
 1. A multilayer coating film formation processcharacterized by that in a process of forming a multilayer coating filmby successively coating with an undercoat paint (A), an intermediatepaint (B) and a topcoat paint (C) on a substrate, (1) to use a liquidthermocurable paint containing 0.1-30 parts by weight of aluminiumpowder and 1-200 parts by weight of titanium oxide pigment per 100 partsby weight of a thermosetting resin composition and the hiding filmthickness of its coating film being less than 25 μm as said intermediatepaint (B), (2) to use a solid color paint, a metallic paint or aninterference pattern paint as said topcoat paint (C), and (3) to coatsaid topcoat paint (C) after curing by heating of the coating film ofsaid intermediate paint (B).
 2. The process set forth in claim 1,wherein the aluminium powder contained in the intermediate paint (B) hasan average particle diameter of less than 40 μm.
 3. The process setforth in claim 2, wherein the aluminium powder has an average particlediameter of less than 10 μm.
 4. The process set forth in claim 1,wherein the titanium oxide pigment contained in the intermediate paint(B) has an average particle diameter of less than 5 μm.
 5. The processset forth in claim 1, wherein the intermediate paint (B) contains 1-7parts by weight of aluminium powder and 80-120 parts by weight oftitanium oxide pigment per 100 parts by weight of the thermosettingresin composition.
 6. The process set forth in claim 1, wherein theintermediate paint (B) contains 1-15 parts by weight of aluminium powderper 100 parts by weight of titanium oxide pigment.
 7. The process setforth in claim 1, wherein the hiding film thickness of the coating filmof the intermediate paint (B) is less than 10-25 μm.
 8. The process setforth in claim 1, wherein the intermediate paint (B) is coated so thatthe film thickness becomes in a range of 10-25 μm based upon the curedcoating film.
 9. The process set forth in claim 1, wherein the coatingfilm of the intermediate paint (B) is cured by heating at temperaturesof about 140 to about 200° C.
 10. The process set forth in claim 1,wherein the topcoat paint (C) is coated so that the film thicknessbecomes in a range of 10-60 μm based upon the cured coating film. 11.The process set forth in claim 1, wherein the coating film of thetopcoat paint (C) is cured by heating at temperatures of about 120 toabout 180° C.
 12. The process set forth in claim 1, wherein theundercoat paint (A) is a cationic electrodeposition paint.
 13. Theprocess set forth in claim 1, wherein a clear paint (D) is furthercoated on the coating surface of the topcoat paint (C).